126
Chapter 8: Longer Term Responses: From Seasons to Centuries
Water to lie adjacent to the shelf edge, and cross-shelf incursions of this carry abundant
Calanus recruits and also render water in the basins and Gulf of Maine warmer and
saltier than during negative NAO conditions.
After 1965, the long-term trend toward increasing positive values of the NAO was
reestablished. The poleward extension of cod populations was reversed, their reproductive
success faltered, and the populations moved progressively southward, even as winters
became harsher, with increased seasonal ice cover and nor’westerly wind stress. The
spring bloom and zooplankton population maxima now occurred progressively later
each year. By the end of the period, cod had been so reduced by environmental change
and continued fishing pressure that the stocks on the Grand Bank had collapsed and
the great ancestral fishery off Newfoundland was closed. The failure of cod to maintain
a population, as they were able to do during earlier sieges of strongly positive NAO
conditions earlier in the century, has been attributed to the fishing-induced truncation of
their population age structure. If insufficient fish are permitted to survive sufficiently long
to bridge each period before conditions became once more favorable for reproduction,
then the stock must collapse, as indeed happened in 1992 (Longhurst, 2002).
In the Trade Wind biome, where reversing monsoon winds and monsoon currents
dominate the regional environment, we are used to strong interdecadal variability in the
strength and reliability of the monsoon regime. These are some of the factors that force
drought, famine, and warfare. Although, as I have suggested, it was the fickleness of the
Indian monsoons that led to the uncovering of the Southern Oscillation Index, the SOI
is now mostly applied to studies of the ENSO phenomenon. But monsoon variability
in the Indian Ocean is expressed principally at much longer time scales that can be
examined in the excellent records that are available for most of the 20th century. These
deliver information not only about the variability of monsoon winds but also about their
oceanographic and biological consequences (Longhurst and Wooster, 1990). As shall be
discussed in Chapter 10 (see INDW), the southwest monsoon forces upwelling on the
west coast of India and this process, in turn, produces diatom blooms and supports a
large population of Sardinella longiceps, the oil sardine, which feeds on them. Because
S. longiceps is a very short-lived species whose abundance responds rapidly to year-to-year
changes in ocean conditions, fishery records can tell us much about long-term changes
in circulation and upwelling.
However, we also have direct information because, used with appropriate caution
for steric effects, sea level is an indicator of upwelling along the Indian coast, as it
is elsewhere. Long time series can be constructed for several stations, and those from
Cochin and Mangalore are particularly useful. Average seasonal variation of the sea level
at Cochin was calculated for the period 1939–1987; we can compare sea level for the
monsoon period of locally forced Ekman divergence (May–September) with sea level for
months when remotely forced, baroclinic upwelling occurs (March–April). The two kinds
of upwelling seem to be somewhat independent because the strongest decadal trends
occur in the April data, which show a trend of increasing sea levels beginning about 1942
and reaching its maximum around 1960. After this, a slightly lower level was sustained
until the end of the data set. In contrast, for the monsoon months (exemplified by June)
decadal trends are less conspicuous, but there is an important change of state from weak
to strong upwelling over a period of 2 or 3 years culminating in 1960. Somewhat similar
trends occur in a longer data set (1878–1982) for Bombay, which shows rising sea levels
from about 1930 until the mid-1950s: it is unlikely that the occurrence of highest sea
levels along western India in the 1950s at two such distant stations could be a matter of
chance.
Comparison of the trends in Cochin sea level and oil sardine catches confirms the
anticipated relationship. A long period of relatively low but variable abundance from 1900
until 1959 was transformed by a massive population increase in 1960, coincident with the
Chapter 8: Longer Term Responses: From Seasons to Centuries
Water to lie adjacent to the shelf edge, and cross-shelf incursions of this carry abundant
Calanus recruits and also render water in the basins and Gulf of Maine warmer and
saltier than during negative NAO conditions.
After 1965, the long-term trend toward increasing positive values of the NAO was
reestablished. The poleward extension of cod populations was reversed, their reproductive
success faltered, and the populations moved progressively southward, even as winters
became harsher, with increased seasonal ice cover and nor’westerly wind stress. The
spring bloom and zooplankton population maxima now occurred progressively later
each year. By the end of the period, cod had been so reduced by environmental change
and continued fishing pressure that the stocks on the Grand Bank had collapsed and
the great ancestral fishery off Newfoundland was closed. The failure of cod to maintain
a population, as they were able to do during earlier sieges of strongly positive NAO
conditions earlier in the century, has been attributed to the fishing-induced truncation of
their population age structure. If insufficient fish are permitted to survive sufficiently long
to bridge each period before conditions became once more favorable for reproduction,
then the stock must collapse, as indeed happened in 1992 (Longhurst, 2002).
In the Trade Wind biome, where reversing monsoon winds and monsoon currents
dominate the regional environment, we are used to strong interdecadal variability in the
strength and reliability of the monsoon regime. These are some of the factors that force
drought, famine, and warfare. Although, as I have suggested, it was the fickleness of the
Indian monsoons that led to the uncovering of the Southern Oscillation Index, the SOI
is now mostly applied to studies of the ENSO phenomenon. But monsoon variability
in the Indian Ocean is expressed principally at much longer time scales that can be
examined in the excellent records that are available for most of the 20th century. These
deliver information not only about the variability of monsoon winds but also about their
oceanographic and biological consequences (Longhurst and Wooster, 1990). As shall be
discussed in Chapter 10 (see INDW), the southwest monsoon forces upwelling on the
west coast of India and this process, in turn, produces diatom blooms and supports a
large population of Sardinella longiceps, the oil sardine, which feeds on them. Because
S. longiceps is a very short-lived species whose abundance responds rapidly to year-to-year
changes in ocean conditions, fishery records can tell us much about long-term changes
in circulation and upwelling.
However, we also have direct information because, used with appropriate caution
for steric effects, sea level is an indicator of upwelling along the Indian coast, as it
is elsewhere. Long time series can be constructed for several stations, and those from
Cochin and Mangalore are particularly useful. Average seasonal variation of the sea level
at Cochin was calculated for the period 1939–1987; we can compare sea level for the
monsoon period of locally forced Ekman divergence (May–September) with sea level for
months when remotely forced, baroclinic upwelling occurs (March–April). The two kinds
of upwelling seem to be somewhat independent because the strongest decadal trends
occur in the April data, which show a trend of increasing sea levels beginning about 1942
and reaching its maximum around 1960. After this, a slightly lower level was sustained
until the end of the data set. In contrast, for the monsoon months (exemplified by June)
decadal trends are less conspicuous, but there is an important change of state from weak
to strong upwelling over a period of 2 or 3 years culminating in 1960. Somewhat similar
trends occur in a longer data set (1878–1982) for Bombay, which shows rising sea levels
from about 1930 until the mid-1950s: it is unlikely that the occurrence of highest sea
levels along western India in the 1950s at two such distant stations could be a matter of
chance.
Comparison of the trends in Cochin sea level and oil sardine catches confirms the
anticipated relationship. A long period of relatively low but variable abundance from 1900
until 1959 was transformed by a massive population increase in 1960, coincident with the